Advances in semiconductor processes technology have produced extremely small transistors. These tiny transistors have thin oxide and insulating layers that can easily be damaged by relatively small currents with even a moderate driving force (voltage). Special care is required when a person handles these semiconductor devices.
Static electricity that normally builds up on a person can discharge across the input pins or a semiconductor integrated circuit (IC or chip). IC chips are routinely tested for resistance to such electro-static-discharges (ESD) using automated testers that apply a voltage across different pairs of pins of the chip. Any pair of pins may be chosen for the ESD test.
Some IC's today include both analog and digital subsystems. The digital portion of a chip often produces switching noise that could be coupled to the analog portions of the chip, resulting in loss of analog precision. Such mixed-signal chips typically use separate power supply busses for analog and digital portions. The internal analog power-supply is separated from the internal digital power-supply, even when both power supplies have the same nominal voltage. Separate pins of the chip apply external power-supplies to the internal analog and digital power-supply busses.
Sometimes several different internal power-supply busses are used for different digital or analog blocks. The output drivers may use a supply bus that is isolated from internal analog and digital supply or ground busses. Some circuits may require a different power-supply voltage, such as for analog circuits or programming circuits. Thus many different, isolated power-supply busses may exist on a mixed-signal IC.
Although these separate internal busses reduce noise coupling, ESD protection is hindered. An ESD pulse applied to any 2 pins may not directly connect to the ground or power supply attached to an input-protection device for a pin. For example, an ESD pulse applied to an analog-input pin and to the digital ground does not necessarily activate the ESD protection device for the analog-input pin, since the protection device might be connected to the analog ground rather than to the digital ground.
Various schemes have been proposed to provide ESD protection for mixed-signal chips. Nguyen et al., in U.S. Pat. No. 5,616,943, discloses using thick-oxide transistors and diodes. See also Puar, U.S. Pat. No. 5,287,241, and Stackhouse et al., U.S. Pat. No. 5,740,000.
FIG. 1A shows a simplified prior-art ESD protection device using a thick-oxide transistor. Power-supply A and B are connected by thick-oxide transistor 12, which does not normally conduct so that the supplies are isolated. However, when a high voltage is applied to supply A during an ESD event, thick-oxide transistor 12 conducts, either by the high voltage forming a conducting channel under the thick field-oxide under the gate, or by punch-through in the substrate from drain to source in the substrate.
While such thick-oxide transistors are less sensitive to damage than thin-oxide transistors, the amount of current conducted is reduced. A very high gate voltage is needed to turn on the transistor since the channel is separated from the gate by the larger distance of the thick oxide. Damage to other thin-oxide transistors on the chip can occur before the thick-oxide transistor turns on. Thus the protection provided by thick-oxide transistor 12 is less than desired.
FIG. 1B shows a simplified prior-art ESD protection circuit that uses diodes. Diodes 14 turn on when the voltage on supply A is sufficiently above the voltage on supply B. Diodes can be formed using diffusion regions or well regions in the semiconductor substrate. These substrate diodes may cause unwanted latch-up when triggered. When diodes 14 are Zener diodes, additional processing steps may be needed, increasing the cost of the chip. When supplies A and B have different nominal voltages, diode chains may not be effective or may require many diodes in series.
Such diodes and thick-oxide transistors are passively controlled, being activated by high voltages during an ESD pulse. The diodes in particular may turn on when the chip is being powered up since they are not actively disabled during normal operation. Such passively-controlled protection devices do not take advantage of active-transistor technology.
What is desired is an ESD-protection circuit that protects internal power supplies in a mixed-signal IC. An active rather than a passive protection circuit is desired. It is desired to actively enable or disable the ESD-protection circuit. It is desired to actively enable and disable a thin-oxide transistor as an ESD shunt between power supply busses. It is desired to avoid thick-oxide transistors and diodes.